The Fischer-Tropsch process can be used for the conversion of synthesis gas into liquid and/or solid hydrocarbons. The synthesis gas may be obtained from hydrocarbonaceous feedstock in a process wherein the feedstock, e.g. natural gas, associated gas and/or coal-bed methane, heavy and/or residual oil fractions, coal, biomass, is converted in a first step into a mixture of hydrogen and carbon monoxide. This mixture is often referred to as synthesis gas or syngas. The synthesis gas is then fed into a reactor where it is converted in one or more steps over a suitable catalyst at elevated temperature and pressure into paraffinic compounds and water in the actual Fischer-Tropsch process. The obtained paraffinic compounds range from methane to high molecular weight modules. The obtained high molecular weight modules can comprise up to 200 carbon atoms, or, under particular circumstances, even more carbon atoms. Numerous types of reactor systems have been developed for carrying out the Fischer-Tropsch reaction. For example, Fischer-Tropsch reactor systems include fixed bed reactors, especially multi-tubular fixed bed reactors, fluidised bed reactors, such as entrained fluidised bed reactors and fixed fluidised bed reactors, and slurry bed reactors such as three-phase slurry bubble columns and ebulated bed reactors.
Catalysts used in the Fischer-Tropsch synthesis often comprise a carrier-based support material and one or more metals from Group 8-10 of the Periodic Table of Elements, especially from the cobalt or iron groups, optionally in combination with one or more metal oxides and/or metals as promoters selected from zirconium, titanium, chromium, vanadium and manganese, especially manganese. Such catalysts are known in the art and have been described for example, in the specifications of WO 9700231A and U.S. Pat. No. 4,595,703.
One of the limitations of a Fischer-Tropsch process is that the activity of the catalyst will, due to a number of factors, decrease over time. The activity of the catalyst is decreased as compared to its initial catalytic activity. The initial activity of the catalyst can be its activity when fresh prepared. A catalyst that shows a decreased activity after use in a Fischer-Tropsch process is sometimes referred to as deactivated catalyst, even though it usually still shows activity. Sometimes such a catalyst is referred to as a deteriorated catalyst. Sometimes it is possible to regenerate the catalyst. This may be performed, for example, with one or more oxidation and/or reduction steps.
After regeneration, catalysts often show an activity that is lower than the activity of fresh prepared catalysts. Especially after multiple regenerations, it often proofs hard to regain an activity level comparable to the activity of fresh prepared catalysts. In order to be able to use a catalyst for a long time, it thus may be desirable to start a Fischer-Tropsch process with a fresh catalyst that has a relatively high activity.
The use of fresh or rejuvenated catalysts with a relatively high initial activity may have disadvantages. This may especially be the case when the amount of catalyst used in a reactor tube is fixed after loading of the catalyst in the reactor tube. One example of a reactor tube filled with a fixed amount of catalyst is a reactor tube filled with a packed bed of catalyst particles.
In a Fischer-Tropsch process with a catalyst with a relatively high initial activity, the activity of the catalyst is especially high at the start of the process. And, due to the high activity of the catalyst, a lot of water is produced in the Fischer-Tropsch hydrocarbon synthesis, resulting in a high relative humidity at the start of the Fischer-Tropsch process. During Fischer-Tropsch synthesis the relative humidity in a reactor tube may increase to such a level that it accelerates the deactivation of the catalyst during use. During start-up of a Fischer-Tropsch reactor with a very active catalyst, the reaction temperature is typically kept at a relatively low value, e.g. below 200° C., in order to avoid a too high product yield and accompanying high temperature rise due to the exothermic reaction. Without wishing to be bound to any theory, it is believed that especially the combination of relatively low temperature and a relatively high yield results in a high relative humidity in the reactor and therewith in undesired irreversible catalyst deactivation.
Therefore, especially in the start-up phase of a Fischer Tropsch reactor with a catalyst with a relatively high activity, the deactivation of the catalyst takes place at a relatively high rate. Deactivation due to relative humidity proofed to be difficult to reverse. The high initial activity in such a case is thus quickly lost, whereas regeneration procedures have only a limited effect.